1. Field of the invention
The invention relates to creating images by transmitting signals and sensing the effect of objects in the field of view on the signals.
2. Description of the Prior Art
Ultrasonic, or ultrasound, imaging devices depend on signal wavefronts that propagate into the body. A wavefront can be described by connecting points of equal phase at a given point in time to form a surface that is perpendicular to the direction that ray paths propagate. The wavefront emanates from a transducer aperture and is shaped by a combination of the transducer surface shape and phasing of signals from separate elements of the transducer surface. For focused operation, the wavefront converges to a point, except that diffraction prevents perfect convergence. Successive wavefronts act like new sources of the signal that are ever smaller and closer to the focus point. The reverse process similarly applies to reception where signals from a point source cause a wavefront of spherical shape which is ideally received by a spherical surface receiver. The body of knowledge of wave propagation physics is relied on to refine and extend this concept.
Effective system operation requires that signal amplitude along the wavefront be well behaved. Problems arise when attenuation is uneven for different paths such that amplitude variations occur along a wavefront. When a coupling fluid is used to allow waves to travel between a transducer and a body it often happens that uneven attenuation situations are set up. As the wavefront propagates through a medium, where the medium is a combination of coupling fluid and body tissue, different paths can undergo different attenuation processes. Undesirable deviations in amplitude can result. This can change the apparent sidelobe levels. Apparent sidelobes are the actual beam response functions that are caused by the propagation effects in combination with ideal aperture effects. Apparent sidelobes will be simply called sidelobes here. This problem is especially significant for high resolution imaging which depends on large apertures and high frequency signals.
Attenuation means the reduction in signal amplitude other than the reduction that comes from geometric spreading of waves. It is also considered separately from the opposite effect of signal level increase that coming from geometric focusing of waves.
Conventional ultrasound practice tends to involve direct contact of a transducer with the skin. Here the body tissue attenuates signals with uniformity over the ray paths to the degree that body tissue is uniform.
It is known in radio frequency antenna design to control power intensity over the antenna aperture with absorbing materials to achieve the desired radiation pattern. It is also known to taper or weight an aperture, where an aperture is a radiating surface of an array of transducers, to reduce sidelobe response.
It is known in ultrasound clinical practice to couple ultrasound signals from transducers to the body by use of a water stand-off. Hitachi Part Number EZU-WL1 is a water bag attachment where the water volume can be adjusted by a syringe. There is a danger with such water filled accessories because the signals at a focus point are not attenuated for shallow operation as they are naturally for deep operation. Although the power levels can be adjusted, it is an action that could be easily forgotten.
Another accessory for oblique stand-off is Hitachi part number EUP-L53ST which is also water filled.
The disclosure of U.S. Pat. No. 5,902,748 (May 1999) Madsen et al. describes a water bag to couple ultrasonic signals from a transducer to a phantom where the phantom mimicks tissue. This method couples the maximum power to the focal point but may not satisfactorily control sidelobe response for a variety of phantom shapes.
It is also known to use a tank or bath wherein a fluid couples signals between a transducer and a subject of examination. A coupling fluid means that the fluid serves as an ultrasonic transmitting medium. Water is commonly used as the fluid but a variety of other fluids are used to enable signal coupling. Castor oil is known to match fat for speed of propagation, thus preventing refraction at a boundary. Johnson and Johnson baby oil is also known as a good match for breast tissue. Like water stand-offs in clinical practice, this method couples the maximum amount of power intensity to the focal point but it may not necessarily produce the desired control of sidelobe response.
It is known to produce tissue mimicking materials for use in forming ultrasound phantoms. U.S. Pat. No. 5,902,748 (May 1999) Madsen et al. discloses useful recipes for making materials that attenuate and propagate as necessary to represent human body parts.
It is known in manufacturing of composite materials to vacuum bag an assembly to remove air bubbles and cause flexible surfaces to mutually conform to each other.
Subjects of examination by ultrasound are commonly human or animal. Other uses are known in other fields.
Referenced documents, in entirety, are incorporated herein. They contribute to the description of the present invention, but in case of conflict, the present document takes precedence.
A general object is to realize maximum resolution benefits of large aperture, high frequency ultrasonic imaging apparatus by utilizing an attenuation leveling method. This method provides controlled attenuation over propagation paths so that signals are at desired amplitudes over wavefronts. This would be able to accommodate different human tissue types. The same applies to animal tissue.
An object is to provide a signal transmission method that is generally useful in clinical practice or laboratory experimental procedure.
An object is to provide safety in ultrasound imaging without causing undue reduction in level of transmitted signals from transducers.
An object is to provide flexible surfaces that comfortably conform to body parts.
An object is to provide shallow viewing near the skin surface.
An object is to establish a fairing surface that enables effective scanning by transducer motion.
An object is to establish a fairing surface that simplifies control of signal amplitudes that are transmitted from various transducer elements.
An object is to establish a fairing surface that shapes body parts to an acceptable degree so as to enable variations between subjects to be accommodated.
An object is to enable treatment access simultaneously with real time imaging.
An object is to establish a laboratory method where uniform wavefronts are maintained by using attenuating material to fill in paths between body parts and transducers.
An object is to provide pre-compensation to balance attenuation variations over different paths.
An object is to enable sidelobe control using tapered or weighted amplitude distribution functions.
An object is to combine attenuation compensation with lens functions.
An object is to utilize these methods in industrial inspection and other fields that involve wave front propagation.
Further objects of the present invention will become apparent from a consideration of the drawings and ensuing description.
The invented method is called attenuation leveling and the invented apparatus enables this method. The basic method involves use of attenuating fluid to maintain a desired amplitude distribution over the surface of a signal wavefront as the wavefront travels between a transducer and human tissue.
A combination including a fairing surface, a conformal surface, and special coupling fluid is devised to convert a human body surface into a surface that is more amenable to high quality ultrasound imaging.
The present invention is a general method that relates to development of materials disclosed in U.S. Pat. No. 5,902,748 (May 1999) Madsen et al. as the invention of eye glasses relates to development of clear glass. The present invention relates to use of water filled stand-off devices as the development of eye glasses relates to glass windows.
This invention can be described as a drum where one end of the drum has a thin rubber sheet fastened to one end of the cylindrical shell of the drum. The other end of the drum has a sheet of Mylar drawn tightly over the other end. The cylindrical shell of the drum is actually a tapered cylinder, that is, a hollow cone. This forms a container that is filled with attenuating fluid that attenuates at the same rate per cm per MHz. as does the body part that is to be examined. The rubber sheet conforms to the body part. The Mylar sheet remains planar. A transducer operating outside the drum through the Mylar sheet will produce the same signal amplitude at the focus point regardless of its lateral position. A coupling fluid efficiently transfers signals from the transducer to the Mylar sheet surface. This would be in a container formed on the opposite side of the Mylar sheet. Better control of the wavefront will be achieved by making this coupling fluid an attenuating coupling fluid. This will give a capability to move the transducer axially while still maintaining the same power intensity at the focus point and maintaining the quality of the focus.
Detailed features include a vacuum method to improve adherence and coupling between human skin surfaces and the rubber sheet and fluid pressure control. Both the Mylar and the rubber sheet are thin such that they have no effect on signal propagation.
An immediate system application involves a transducer that is mechanically scanned. This system enables breast imaging where a variety of breast sizes and shapes can be accommodated. Mechanical scanning is facilitated by the fairing surface formed by the taut Mylar sheet.
Another variation is a pre-compensating attenuating pad that allows for variations in the fluid in which the transducer is immersed. Pre-compensation is an uneven way to control attenuation because wavefronts are allowed to be uneven in amplitude over different successive positions and this can give rise to scattering effects.
Variations include use of conventional, hand held ultrasound transducers with fluid filled cushions that are thin walled, rubber pouches that are filled from a reservoir or with a syringe. In such cases, the fluid is an attenuating fluid such as evaporated milk. Fluids vary to suit the applicable tissue type. Fluids can be gels or other firm or solid materials as desired.
Safety is improved over conventional water stand-off methods since the power level transmitted can be kept at the maximum level needed for deep penetration.
Other variations exclude the fairing function. These include a fluid bath wherein both a transducer and a subject of examination are immersed and the fluid is an attenuating fluid that enables uniform amplitude wavefront.
The scope of the invention should be determined by the appended claims and their legal equivalents and not by the examples and variations given. Actual medical practice would be expected to result in many variations of this concept.